Electron beam system



May 12, 19-59 PING K. TIEN 2,836,733

ELECTRON BEAM SYSTEM Filed Jan. 29, 1954 2 Sheets-Sheet 1 WWW x \l I QlI mllll" r INVENTOR t K. T/E/V ATTORNEY May 12, 1959 PING K. TIEN2,886,738

ELECTRON {BEAM SYSTEM Filed Jan. 29, 1954 2 Sheets-Sheet 2 o 7 X i V) Z/4 M Li FIG 3 INVENTOR y R K. T/EN A TTORNEV United States Patent iELECTRON BEAM SYSTEM Ping K. Tien, Chatham Township, Morris County, NJ.,assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Application January 29, 1954, Serial No. 406,922

12 Claims. (Cl. 315-3.6)

This invention relates to electron beam systems, and more particularlyto systems for focusing an annular electron beam over a relatively longpath.

One object of the present invention is to facilitate the focusing ofannular electron beams.

Typically, the invention finds application in electronic devices such astraveling wave tubes in which an electron beam is made to flow past awave interaction circuit for a distance a plurality of operatingwavelengths long whereby the beam and a wave propagating along thecircuit interact, the beam being velocity and density modulated and thewave being amplified. In such devices, it is desirable to keep the beamflowing close to the circuit where the electric field components of thewave are high and yet to avoid having the beam impinge on the circuit,since such impingement results in a loss of efficiency and noisyoperation, and may even damage the circuit. Such flow is mostconveniently achieved if the electron beam is kept cylindrical. However,in an electron beam, space charge forces are present which tend to makeit diverge. Accordingly, it is generally advantageous to exert on thebeam focusing action of some kind to overcome the space charge forces ifthe beam is to be kept cylindrical.

Various expedients have been utilized hitherto to provide this focusingaction. Early arrangements employed a uniform longitudinal magneticfield along the path of flow for focusing the beam. However, except atthe shorter wavelengths where a short electron path is sufficient, theauxiliary equipment needed to create such a field has generally been ofsuch weight and size as to make this unattractive. Accordingly, effortshave continuously been directed at alternative arrangements. Among thoseappearing most attractive at first sight are arrangements described in apaper entitled Axially Symmetric Electron Beam and Magnetic FieldSystems, by L. A. Harris, published in the Proceedings of the IRE, June1952, pp. 700-708. Therein it is proposed that an electron beam can befocused by making it annular, giving it a rotational component, and thenprojecting it through an annular passage formed between two coaxialcylindrical electrodes which are maintained at different D.-C.potentials for establishing a radially inward electric field across thepassage. The motion of the electrons is sought to be stabilized bymaintaining a balance between the radially outward centrifugal forcesarising from the rotational component of the annular beam, the radiallyoutward space charge forces in the annular beam, and the radially inwardforces set up by the radial electrostatic field between the twocylindrical electrodes defining the beams annular path.

However, it develops that this technique, although possessing certaindesirable characteristics, has several shortcomings.

First, it is found that establishing a potential difference between thetwo cylindrical electrodes complicates the potential distribution insidethe gun structure, which structure is necessarily positioned in a regionadjacent the cylindrical electrodes, and as a result a distortion of thePatented May 12, 1959 electron trajectories in this preliminary regionof the beam path is difiicult to avoid.

Additionally, another problem arises in maintaining the electrondynamics in a stable condition across the entire cross section of thebeam. For an understanding of this problem, consider the followinganalysis of the forces on the beam. The radially inward force acting oneach electron as a result of the radial electrostatic field decreasesgradually with increasing radial distance because of the increasingseparation from the more posi tively charged inner electrode. Theradially outward centrifugal force acting on each electron because ofits orbital motion decreases gradually with increasing radial distancesince the rotational component is imparted by a radial magnetic fielddensity which decreases with increasing radial distance whereby theouter electrons are given a slower angular velocity than the innerelectrons. The radially outward space charge force in a beam of uniformcurrent density increases sharply with increasing radial distance. As aresult, in such a beam it would be difficult to maintain a balance overits entire cross section. To avoid this problem, in such prior artarrangements, the annular beam is made to have a current density whichdecreases sharply with increasing radial distance whereby the spacecharge forces increase only gradually with increasing radial distance.The requirement of such a current density distribution limits theavailable electron gun structures which can be profitably employed andmakes the design of the gun structure difiicult.

Moreover, another shortcoming of such prior arrangements is that at theshorter wavelengths where the physical dimensions of the various tubeelements are necessarily quite small it becomes a difiicult practicalmatter to provide an electrode within the annular path of flow toestablish radial electrostatic fields.

The present invention relates to basic modifications of theabove-described arrangement which largely obviate the diflicultiesdiscussed.

In accordance with one feature of the invention, along the initialportion of the path of flow there is positioned a magnetic fluxproducing means adapted to provide a region of longitudinal magneticfield followed by a region of radial or transverse magnetic fields. Theelectron source is positioned in the region of longitudinal magneticfield whereby the electron beam is magnetically confined for the initialportion of its path of travel. Thereafter, the electron beam beforeentering into a region of spatially alternating field passes through theregion of radial magnetic field where it is given a rotationalcomponent. Such an arrangement serves to minimize radial components inthe annular beam preliminary to its injection into a region of spatiallyalternating fields and to provide rotational components to the electronsin the beam. Additionally, in contradistinction to the prior artarrangements described the radial magnetic field density is adjusted toincrease with increasing radial distance whereby the outer electrons inthe beam are given a larger rotational component. As a result, theradially outward centrifugal forces present in the beam increasegradually with increasing radial distance.

Another feature of the invention is the combination of a spinningannular beam with a time-constant spatially alternating longitudinalfield, either electric or magnetic. Such a spatially alternating fieldcan be made to apply radially inward forces on an electron beam whichare appreciably larger on the outer electrons of the beam than on theinner electrons. By utilizing such a spatially alternating field, theradially inward electrostatic forces resulting therefrom, the radiallyoutward centrifugal forces and the radially outward space charge forcesmay be balanced over all of the cross section of the beam,

even in cases where the electron beam is of substantially uniformcurrent density over the cross section of the beam and so where thespace charge forces increase sharply with radial distance. Thissimplifies the design of a suitable electron gun.

A further advantage in such novel combinations which utilize a spatiallyalternating longitudinal field in combination with an annular beamhaving a rotational component is the elimination of the need for anaxially displaced electrode within the annular beam since such a fieldmay be achieved without any inner electrode. in such a combination theinner boundary of the annular beam is maintained as a result of thebalance between the outwardly directed centrifugal force of the spinningelectrons and the inwardly directed force occasioned by the spatiallyalternating focusing field. The outwardly directed space charge force atthe inner boundary of the annular beam is ordinarily small enough to beneglected. This elimination of the necessity for an axially disposedelectrode results in a simplification of the fabrication problems.

As indicated above, the spatially alternating field can be eithermagnetic or electrostatic. An electrostatic field f the kind desiredcan, for example, be obtained by utilizing a bifilar helix Whose twoconductors are maintained at different D.C. potentials. Such a bifilarhelix can advantageously be employed simultaneously as the wave circuitfor propagating the electromagnetic wave for interaction with the beam.Alternatively, a spatially alternating magnetic field of the kinddesired can be achieved, for example, by spacing along the path of flowa series of annular magnetic members for serving as pole pieces andpoling successive members oppositely.

Various electron gun structures can be utilized for providing theannular electron beam for projection along the region of spatiallyalternating fields. In specific embodiments to be described, annularelectron beams are provided by a hollow-cathode type gun, an electrongun comprising a planar cathode and two apertured planar electrodes, anda so-called Pierce-type gun. Each of these guns is modified from theirusual form to provide annular electron beams.

The invention will be better understood from the following more detaileddescription taken in conjunction with the accompanying drawings inwhich:

Figs. 1 and 2 each show, in longitudinal cross-sectiontional view asillustrative embodiments of the invention, bifilar helix-type travelingwave tubes in which spinning annular beams are focused by spatiallyalternating electrostatic fields; the two tubes differing in that thefirst employs a modified Pierce-type electron gun to provide the annularbeam and the second utilizes a planar cathode and two apertured planarelectrodes; and

Fig. 3 shows, in longitudinal cross-sectional view as anotherillustrative embodiment of the invention, a helixtype traveling wavetube in which a spinning annular beam supplied by a hollow-cathode typegun is focused by a spatially alternating magnetic field.

Referring now more particularly to the drawings, in the traveling wavetube shown in Fig. 1, an evacuated glass envelope 11 houses the varioustube elements. At one end of the envelope, an electron gun 12 of thetype generally designated as the Pierce-type serves as the electronsource for providing an annular beam of substantially uniform densityover its cross section. The design considerations of such a gun aredescribed in detail in United States Patents 2,268,196 and 2,268,197which issued on December 30, 1941, to J. R. Pierce. The gun comprisesthe annular cathode 12A, the annular beamforming electrode 12B, and theannular accelerating an ode 12C. Magnetic materials are avoided in thegun. Consistent with the principles set forth in the above-identifiedpatents, the surfaces of electrode 12B and anode 12C are of suchconfiguration that therealong in the presence of complete space chargethe electric field perpendicular to a line extending normal to theemissive surface of cathode 12A is zero. The electrode 123 and the anodeeach include a central or inner portion and an outer portion definingannular passages therethrough for the annular beam. Each of these innerportions is advantageously planar normal to the path of fioW. By theinclusion of such portions, the usual form of Pierce gun may beadvantageously modified for providing an annular beam of uniform currentdensity. In operation, the cathode 12A is maintained at referencepotential, the electrode 128 at the potential slightly negative thereto,and the anode 12C at a positive potential thereto. In the interest ofsimplicity lead-in connections to such elements have not been shown.

At the opposite or downstream end of the envelope in target relationshipwith the gun is the collector 13 which collects the spent electrons.Throughout the specification the term downstream is used to denote apoint or location closer to the collector than the point or locationwith which it is being compared. Conversely, the term upstream denotes apoint or location closer to the electron gun than the point with whichit is being compared. Intermediate the gun and the collector along thepath of flow is positioned a bifilar helix comprising helical ribbonconductors 14 and 15. The helical conductors are supported by the Wallsof the tube envelope and aligned to be coaxial with the annular electronbeam Whose path of flow is surrounded by the conductors. In order toachieve a spatially alternating electrostatic field in accordance withone characteristic of the invention, the two helices are maintained atdifferent D.C. potentials positive with respect to the cathode 12A bysuitable leadin connections (not shown for the sake of simplicity).

Additionally, a magnetic fiux producing means, such as the solenoid 21is positioned adjacent the source or upstream end of the path of flow.Such means serves a dual purpose. First, it provides a longitudinalmagnetic field in which the electron gun is immersed to confine the beamfrom the time when it is first emitted from the cathode and until it isto be confined by the periodic fields. Also, it provides a transversecomponent of magnetic field just before the beam enters the region ofperiodic electrostatic field in order to impart a rotational componentto the beam. In this arrangement, since the density of the radialmagnetic field increases with radial distance from the axis, the outerelectrons will get a faster angular rotation and so have a largercentrifugal force acting on them. To shape the magnetic flux to thedesired configuration, two annular disks 16 and 17, of highly permeablematerial such as kovar, are positioned transverse to the path of flow,extending as pole pieces from the solenoid. In the arrangement shown,the disk 16 is external and disk 17 internal the tube envelope. Byproperly positioning these pole pieces the magnetic flux linking the twothrough the air gap is shaped to be longitudinal when it passes throughthe cathode and the annular openings in the beam shaping electrode andaccelerating anode, and to be transverse at the region where theelectron beam passes the disk 17 and enters into the region ofalternating field. In Fig. 1, atypical magnetic fiux pattern is shown bythe broken lines.

The magnetic disk 17 along the path of flow is preferably maintained, bya lead-in connection not shown, at the potential of one of the twohelical conductors, for example the conductor 15, and so cooperatestherewith for forming the first half period of the alternating field.The average potential of the two helical conductors with respect to thecathode determines the longitudinal velocity of the beam past theconductors 14 and 15. The two conductors preferably are also ofnon-magnetic ma terial.

in operation, electrons emitted from the cathode 12A are formed into anannular beam by the action of the beam forming electrode 12B andaccelerated towards the accelerating anode 12C. The electrons areconfined to longitudinal paths with a minimum of transverse componentsin following the longitudinal magnetic lines through this gun region. Asthey approach the pole piece 17, where the magnetic flux acquires atransverse direction, they are given a rotational component which setsthem spinning with an angular velocity which is determined by thestrength of the magnetic field. Once beyond the region of transversemagnetic field, the spinning electron beam is focused by the spatiallyalternating electrostaticfield existing therebeyond. A spatiallyalternating field of this kind can be shown to provide a net convergenceforce on the beam. A quantitative analysis of the efiect of such a fieldis set forth in my U. S. Patent 2,843,776, issued July 15, 1958.Moreover, it is found that this force increases sharply with increasingradial distance from the axis of the bifilar helix up to a radius equalto the radius of the bifilar helix.

The bifilar helix formed by conductors 14 and 15 is well adapted forserving as a wave circuit for propagating waves for interaction with theelectron flow. Such a circuit may be operated either in a forward wavemode, as is characteristic of the usual traveling wave amplifier, or ina backward wave mode as is characteristic of a backward wave oscillatorand backward wave amplifier. The general principles of both such typesof operation are set forth in my above-mentioned patent.

For purposes of illustration, the tube shown is designed for operationas a forward wave amplifier. To this end, the input wave to be amplifiedis applied to the electron source end of the bifilar helix and theoutput wave is abstracted at the collector end. Input and output wavecoupling connections 18 and 19, respectively, are shown schematicallyfor purposes of simplicity. A suitable arrangement for couplingrectangular wave guide connections to such a bifilar helix travelingwave tube is described in U.S. Patent 2,846,613, issued August 5, 1958,of J. R. Pierce.

Various modifications of this described embodiment are possible. It may,for example, be advantageous for higher density electron beams toutilize an electron gun which provides conical flow initially. In thisinstance, the pole pieces associated with the flux producing means areshaped and positioned to provide in the region be tween the cathode andthe start of the periodic field a confining magnetic field which hasboth a longitudinal and transverse component. Additionally, the beamfocusing electrode and accelerating anode forming the gun electrodesystem similarly should have surface configurations suitable forcooperating with the magnetic field in keeping the flow uniformlyconical preliminary to its being made cylindrical for flow through theregion of periodic fields. Various forms of electron guns can beemployed. There is described hereinafter tubes incorporating alternativeforms of electron guns. Additionally, the periodic electrostatic fieldcan be achieved by a succession of spaced cylindrical disks maintainedat appropriate potentials and surrounding the path of flow. In such aninstance, an interaction circuit separate from the focusing structure,such as a single wire helix, may be employed to propagate the travelingWave for interaction with the flow.

In Fig. 2 there is shown a bifilar helix-type traveling wave tube 30difiering from tube 10 shown in Fig. 10 essentially only in the electrongun structure. The same reference numerals are used to designatecorresponding elements, and accordingly only the electron gun referencenumerals are different. Tube 30 incorporates an electron gun 31 whichcomprises a planar annular cathode 31A and two spaced annularlyapertured planar electrodes 31B, 310, each having coplanar inner andouter portions. The gun 31 is immersed in a longitudinal magnetic fieldprovided between the pole pieces 16 and 17 associated with solenoid 20.Each of the two apertured electrodes 31B, 31C serves as an electronlens. As is described in greater detail in an article by J. R. Pierceentitled A Gun for Starting Electrons Straight in a Magnetic Field, involume 30, page 825 of the Bell System Technical Journal, by properlychoosing the spacing between and the potentials of electrodes 31B and31C the electron beam emerging beyond the electrode 31C can be made tohave no radial components. For use with such an electron gun, the polepiece 17 can advantageously be provided with inner and outer portionsdefining an annular opening for passage of the annular beam. As withtube 10, the potential of this pole piece can be chosen so that the polepiece can cooperate with the bifilar helix in forming the periodicelectrostatic field.

As has been indicated above, the spinning annular electron beam can befocused in accordance with the principles set forth by utilizing atime-constant spatially alternating magnetic field in place of thetime-constant spatially-alternating electric field. In Fig. 3 there isshown, as another illustrative embodiment of the invention, a travelingwave tube 40 utilizing a periodic magnetic field along the path of flowfor focusing the spinning annular beam. At one end of the evacuatedglass envelope 41, a hollow cathode-type electron gun 42 serves as thesource of an annular electron beam which is directed along the tube axistowards a target 43 at the opposite end which collects the spentelectrons. The electron gun comprises essentially a cathode assembly 44and an intensity control element 45. The cathode assembly 44 includes ahollow sphere 46, for example of nickel, which is enclosed except for acircular orifice or aperture 47. The internal surface of the sphere iscoated with a layer 48 of electron emissive material. Electron emissionfrom the layer 48 exits via the orifice 47 which communicates betweenthe hollow interior of the sphere and the exterior. A heater 49 ismounted in heat transfer relation with the sphere. The heater 49 and thesphere 46 are enclosed within a heat shield 50 to insure efiicientheating. The electrode 45 is positioned adjacent the sphere opposite theorifice 47. The electrode 45, for example, is a mesh grid supported bymounting means not here shown. The beam intensity is controlled by thepotential of this electrode. The various lead-in connections have beenomitted in the interest of simplicity. Hollow-cathode type guns of thiskind are described in more detail in U.S. Patent 2,810,088, issuedOctober 15, 1957, of D. MacNair.

The sphere 46 has in the region of the orifice 47 an appreciable wallthickness. The orientation of the wall surface 51 of the orifice isimportant in controlling the direction of electron emission. Forachieving a cylindrical annular beam, the wall surface 51 should beeverywhere parallel to the direction of desired flow. The relationshipbetween the diameter D of the orifice 47 and the length L of the wallsurface 51 largely determines the current distribution across the beam.For sufiiciently large values of D/L the emission is substantiallyannular. Additionally, aligned with the orifice 47 and the intensitycontrol electrode 45 there is supported an accelerating anode 52.

In accordance with one characteristic of the invention, to maintain thebeam cylindrical and to impart a rotational component thereto, theelectron gun 42 is immersed in a magnetic field. To this end, there ispositioned, preferably external to the tube envelope 41, flux producingmeans, such as a solenoid 54, with which are associated two annularmagnetic disks 55, 56 for serving as pole pieces as described inconjunction with the tube shown in Fig. 1. The magnetic fieldconfiguration is adjusted by properly positioning and dimensioning suchpole pieces 55, 56 so that the field is longitudinal through the cathodeassembly 44 and past the intensity control electrode 45 and acceleratingelectrode 52 and becomes trans- 56. This pole piece is preferablymaintained at the same positive potential to be used on the waveinteraction circuit for establishing the longitudinal velocity of thebeam therepast.

The spinning electron beam is thereafter focused by a time-constantspatially alternating magnetic field. To this end, a succession ofannular elements 61, of a magnetic material such as soft iron, arespaced apart along the path of flow and surrounding the tube envelopefor serving as pole pieces. A succession of annular cylindricalpermanent magnets 62, magnetized in an axial direction, are positionedalong the path of flow for bridging successive pairs of pole pieces 61.Successive magnets are reversed in sense as shown whereby successivepole pieces are poled oppositely. In this way, a spatially alternatingmagnetic field can be achieved along the path of fiow which will providea net converging force on the annular beam. Various other arrangementsfor achieving a suitable spatially alternating magnetic field, togetherwith an analysis of the converging forces provided by such fields, arefound in U.S. Patent 2,847,- 607, issued August 12, 1958, of I. R.Pierce.

In a tube of this kind, the wave transmission circuit can be chosenindependent of the focusing requirements. In the tube shown, a singlewire helix 63 is used as the interaction circuit. Such a helix probablyforms the most common type of interaction circuit used in traveling wavetubes and its operation is now well understood by workers in the art.The helix is supported by the glass envelope and is coaxial with theannular beam which flows closely past its interior surface. The helixpotential is adjusted by means not shown to provide an acceleratingfield on the beam which gives it a longitudinal velocity suitable forinteraction with the traveling wave. IvIoreover, the winding pitch ofthe helix provides a measure of control of the phase velocity of thebeam. In choosing the helix pitch and helix potential suitable foroptimum interaction, a suitable correction for the angular velocity ofthe electron beam will probably need to be made. Input and output wavecoupling connections to the helix circuit, shown schematically here, canbe of conventional form, suitably modified for the presence of themagnetic structure. Arrangements which permit extending the spatiallyalternating magnetic fields as close to the electron source as desiredin the presence of input and output connections are described in theaforementioned Pierce patent. In the tube shown, a permanent magnet canbe bridged between the pole piece 56 and the first of the pole pieces 61to start the periodic longitudinal magnetic field in the beam pat-hregion immediately beyond the pole piece 56.

It is to be understood that the various embodiments which have beenshown are merely illustrative of the general principles of theinvention. Various modifica tions Will be evident to workers skilled inthe art without departing from the spirit and scope of the invention.For example, it should be obvious that the focusing principles of theinvention can be utilized in electronic devices, other than travelingwave tubes, which utilize relatively long electron beams whose fiowshould be kept cylindrical.

What is claimed is:

1. In an electron beam system, an annular electron source and a targetelectrode defining therebetween a path of flow for an annular electronbeam, means adjacent the source end of said path for confining theelectron beam and imparting a rotational component thereto, and meansdownstream along the path with respect to said first mentioned means forforming a field which is constant with time and spatially alternates indirection along the path of flow for focusing the annular electron beam.

,2. In an electron beam system, an electron source and a targetelectrode for defining therebetween a path of flow for an annularelectron beam, flux-producing means adjacent the electron source end ofthe path for forming a longitudinal magnetic field along the initialportion of the path of flow for confining the electron beamand r; so

forming at a region downstream from said initial portion a transversemagnetic field across the path of flow for imparting a rotationalcomponent to the electrons in the beam as they travel therethrough, andmeans along the path of flow downstream with respect to saidfluxproducing means for providing a field which is constant with timebut spatially alternates in direction along the path of flow forfocusing the annular electron beam.

3. In combination, an electron source and a target electrode for forminga path of flow of an annular electron beam, flux-producing meansadjacent the upstream end of the path of flow for confining the electronbeam in the initial portion of its path of flow and imparting arotational component to the beam at a subsequent point in its path offlow, and periodic focusing means for forming a periodic field having asuccession of field regions along a portion of path of flow downstreamof said subsequent point for focusing the electron beam, the sense ofadjacent field regions of the succession being opposite.

4. An electron beam system according to claim 3 in which the periodicfocusing means includes a plurality of spaced annular magnetic memberssurrounding the path of flow and permanent magnet means for polingsuccessive members oppositely.

5. In combination, an electron source and a target for definingtherebetween a path of flow of an annular elec tron beam, flux-producingmeans positioned upstream along the path of flow for providing alongitudinal magnetic field along the initial portion of the path offlow and downstream therefrom a transverse magnetic field across thepath of flow for imparting a rotational component to the electrons inthe beam, and means downstream from said flux-producing means forforming a succession of electrostatic field regions along the path offlow, the sense of adjacent field regions of the succession beingopposite.

6. An electron beam system in accordance with claim 5 in which thelast-mentioned means includes two helical conductors arranged to form abifilar helix and means forv maintaining different D.-C. potentials onthe two conductors forming the bifilar helix.

7. In a traveling wave tube, means forming a finite region ofunidirectional longitudinal magnetic field and a region of transversemagnetic field, means forming a finite region of spatially alternatinglongitudinal field contiguous with said region of unidirectionallongitudinal magnetic field, an electron gun positioned in the region ofunidirectional longitudinal magnetic field and spaced upstream from theregion of transverse magnetic field for providing an annular electronbeam, and a wave interaction circuit positioned downstream of the regionof transverse magnetic field in the region of spatially alternatingfield for propagating an electromagnetic wave in coupling relation withthe electron beam.

8. A traveling wave tube according to claim 7 in which the electron guncomprises an electron emissive cathode, a beam forming electrode and anaccelerating anode.

9. A traveling wave tube according to claim 7 in which the electron guncomprises a planar cathode and at least two spaced planar electrodesapertured for passage-therethrough of the electron beam.

10. A traveling wave tube according to claim 7 in which the electron gunincludes a spherical hollow cathode.

11. In combination, an electron gun and a target for definingtherebetween a longitudinal path of flow for an annular electron beam,flux-producing means adjacent the electron source end of thelongitudinal path for immersing the electron source and an initialportion of the longitudinal path in a unidirectional longitudinalmagnetic field and for providing a transverse magnetic field across anintermediate portion of the longitudinal path of flow, and means forforming. a field which is spatially alternating in direction along theportion of the path of flow downstream of said intermediate portion.

12. A combination according to claim 11 which includes means forming apath for electromagnetic waves along the portion of the longitudinalpath of flow downstream of said intermediate portion.

Cage Feb. 13, 1940 Lindenblad Oct. 27, 1942 10 De Forest Nov. 22, 1949Pierce Jan. 16, 1951 Hines Aug 26, 1952 Hansell Oct. 21, 1952 Hull Mar.17, 1953 Dewey June 23, 1953 Wang Apr. 10, 1956 Ettenberg July 3, 1956Johnson et a1. Oct. 8, 1957 Kompfner Nov. 5, 1957

